The present invention relates to unimorph- and bimorph-type piezoelectric/electrostrictive elements used as various transducers and various actuators.
A piezoelectric/electrostrictive element is used in various fields such as various transducers for converting electrical energy into mechanical energy, that is, electrical energy into mechanical displacement, force, or vibration, and vice versa, various actuators, frequency-region functional components including various actuators and filters, various display devices including displays, sounding bodies including loudspeakers, microphones, and sensors including ultrasonic sensors.
For example, there are disclosed a piezoelectric/electrostrictive element comprising a film-type piezoelectric/electrostrictive operating portion 5 constituted with a ceramic substrate 1 serving as a diaphragm and a first electrode film 2, a piezoelectric/electrostrictive film 3, and a second electrode film 4 formed on the ceramic substrate 1 (Japanese Patent Application Laid-Open No. 3-128681) as shown in FIG. 1(a), and a piezoelectric/electrostrictive element in which a ceramic substrate has a cavity, the bottom of the cavity is constituted so as to be a thin-wall portion, and a piezoelectric/electrostrictive operating portion is integrated on the outer surface of the thin-wall portion (Japanese Patent Application Laid-Open No. 5-49270) as shown in FIG 1(b).
Moreover, a ceramic substrate using zirconium oxide partially stabilized with yttrium oxide is generally known as the ceramic substrate constituting a piezoelectric/electrostrictive element 7 (Japanese Patent Application Laid-Open Nos. 5-29675, 5-97437, and 5-270912).
In the case of the above piezoelectric/electrostrictive element, however, when forming a piezoelectric/electrostrictive operating portion 5 on the ceramic substrate 1 and firing the piezoelectric/electrostrictive operating-portion 5 in the fabrication process of the piezoelectric/electrostrictive element, a crack maybe recognized at a specific portion, that is, at a portion of ceramic-substrate 1 near the boundary of the first electrode film 2 as shown in FIGS. 2(a) and 2(b) particularly through heat treatment (firing) for integrating the piezoelectric/electrostrictive film 3 with the first electrode film 2 and the ceramic substrate 1 depending on the firing condition, and thereby a problem occurs that the production yield is lowered.
As a result of observing the vicinity of the portion where the crack occurs with an electron-probe microanalyzer (EPMA), it is found that the amount of yttrium oxide serving as a stabilizer for zirconium oxide is small as compared with other portions. Though the reason why the yttrium oxide is decreased is not known, it is estimated that, because the above portion is a portion which piezoelectric/electrostrictive film 3 directly contacts the ceramic substrate 1 when the piezoelectric/electrostrictive film 3 protrudes over the first electrode film 2 on the ceramic substrate 1 in order to prevent a short circuit from occurring between upper and lower electrodes, the yttrium oxide selectively diffuses to the piezoelectric/electrostrictive film 3 when the film 3 is sintered and integrated. Moreover, from the viewpoint of device structure, the vicinity of the boundary of the first electrode film 2 on the ceramic substrate 1 is a portion to which a large stress is easily applied to integrate the piezoelectric/electrostrictive film 3, first electrode film 5, and ceramic substrate 1 through heat treatment. Particularly, when the substrate has a cavity structure as shown in FIG. 1(b) or FIG. 2(b), it is estimated that the stress under the heat treatment causes a decrease in the yttrium oxide because the stress becomes high particularly in the cavity structure. However, this is not clear. In any case, however, it is strongly estimated that crystal phase transformation of zirconium oxide is induced due to a decrease in the yttrium oxide and this results in the crack formation.
To prevent a crack from occurring, it is considered to fabricate the ceramic substrate 1 with fully stabilized zirconium oxide in an attempt to prevent crystal phase transformation. However, fully stabilized zirconium oxide is inferior to partially stabilized zirconium oxide in mechanical strength. Therefore, for example, even when decreasing the thickness of the ceramic substrate 1 in order to improve the displacing characteristic of an actuator or the sensitivity of a sensor, a problem occurs that the thickness of the ceramic substrate 1 cannot be effectively or sufficiently decreased.
Therefore, though the partially stabilized zirconium oxide, particularly the zirconium oxide partially stabilized with 2 to 4 mol % of yttrium oxide, is superior in diaphragmatic characteristic, the partially stabillized zirconium oxide is susceptible to crystal phase transformation and cracking if the amount of yttrium oxide serving as a stabilizer is decreased due to any factor while sintering the piezoelectric/electrostrictive film 3 as described above. Moreover, the above problem is peculiar to a piezoelectric/electrostrictive element constituted by integrating a ceramic substrate serving as a diaphragm with a film-type piezoelectric/electrostrictive operating portion through heat treatment without using an adhesive or the like.